Enhanced oil recovery processes

ABSTRACT

Oil recovery processes from carbonate or sandstone reservoirs. With a carbonate reservoir, the reservoir is initially flooded with a fluid such as sea water. Then the same fluid containing a plurality of citric acid-filled microcapsules is injected into the reservoir. These microcapsules are left to incubate in the reservoir, which will then infiltrate the rock formations, degrade by heat and release the encapsulated citric acid. The released citric acid reacts with the carbonate rocks to produce CO 2  in situ, which causes oil trapped in the rock formations to swell, reduce in viscosity and move towards to a nearby production well. For a sandstone reservoir, calcium carbonate can be also encapsulated with the citric acid for CO 2  generation at the reservoir.

BACKGROUND OF THE INVENTION

Technical Field

The present invention relates to processes and techniques for crude oilrecovery. More specifically, the present invention relates to processesfor enhancing in situ generation of carbon dioxide in an oil reservoirthat in turn results in increased oil recovery.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Crude oil development and production in global oil reservoirs caninclude up to three distinct phases: primary, secondary and tertiary (orenhanced) recovery. During primary recovery, reservoir drive comes froma number of natural mechanisms. These include: natural water displacingoil downward into the well, expansion of the natural gas at the top ofthe reservoir, expansion of gas initially dissolved in the crude oil,and gravity drainage resulting from the movement of oil within thereservoir from the upper to the lower parts where production wells arelocated. Only about 10% (e.g. 5-15%) of a reservoir's original oil inplace is typically produced by the natural mechanisms of primaryrecovery. Secondary techniques extend a field's productive life afterthe natural reservoir drive diminishes, generally by injecting externalenergy in the form of water (e.g. water injection or waterflooding) orgas to increase the reservoir pressure, so that the oil can beartificially displaced and driven to a production wellbore, resulting inthe recovery of 20-40% of the original oil in place.

As global energy demand continues to surge and the amount ofeasy-to-produce oil (by primary and secondary recoveries) diminishesrapidly, oil producers are investing and searching for methods toincrease oil recovery, including the recovery of residual oil from agrowing number of mature oil fields that have already been subjected toprimary and secondary recoveries. The residual oil is usually heavy:having high viscosity and therefore resulting in low oil mobility.

Techniques in enhanced oil recovery (EOR) offer prospects for ultimatelyproducing 30-60%, or more, of the reservoir's original oil in place. EORprocesses attempt increase the recovery factor by focusing on therock/oil/injectant system (e.g. wettability of reservoir rocks) as wellas the interplay of capillary and viscous forces (i.e. to reduce theviscosity and thereby increase the mobility of the oil especially theresidual oil). Three major categories of EOR have been found to becommercially viable to varying degrees: thermal recovery, gas injection(e.g. natural gas, N₂ or CO₂) and chemical injection (e.g. polymerflooding and microbial injection).

The EOR technique that has attracted the most new market interest isCO₂-EOR. In the U.S., CO₂ injection has been implemented through thePermian Basin of West Texas and eastern New Mexico, and is now alsobeing pursued at varying extents in other states such as Kansas,Mississippi, Wyoming, Oklahoma, Colorado, Utah, Montana, Alaska andPennsylvania.

CO₂ is effective in recovering oil from a reservoir because it promotesswelling of the oil, reduces the viscosity and vaporizes portions ofcrude oil as it is being transported through the porous rock. However,as CO₂ is highly mobile, this technique encounters problems of viscousfingering, reservoir heterogeneity and gravity overriding orsegregation, as the ability to control the mobility of CO₂ is limited.

Attempts to reduce the mobility of CO₂ include in situ generation of CO₂in oil reservoirs and injection of CO₂ in a supercritical fluid state oras carbonated water. These strategies can also be accompanied by theinjection of chemicals such as viscosifiers, surfactants and nanosilicaparticles for foam formation.

US 2014/0338903A1 describes the use of HEDTA chelating agents at low pHvalues to generate CO₂ in situ in carbonate cores. This method is goodfor in situ CO₂ generation at near wellbore regions but not for deepplacement and maximum contact with the reservoir as the HEDTA or any lowpH chemical, having no retarding mechanisms for their reaction, will beconsumed at the reservoir.

Gumersky et al. uses a gas-forming solution containing water and amixture of low concentrated acid and low concentrated surfactant andpolymer. The solution forms a stable foam and while penetrating throughbarrier-blocking high permeable layers and into low permeable layers,the gas-forming solution shows visco-elastic properties and displacesoil from them. This method improved water flooding efficiency by 20-30%and the ultimate oil recovery improved marginally by 3-5% compared totraditional water flooding [Gumersky, K., Dzhafarov, I. S.,Shakhverdiev, A. K. and Mamedov, Yu. G. 2000. In-Situ Generation ofCarbon Dioxide: New Way to Increase Oil Recovery. Paper SPE 65170-MSpresented at SPE European Petroleum Conference, Paris, France, 24-25Oct. 2000—incorporated herein by reference in its entirety].

Shiau et al. heat ammonium carbamate above 85° C. to produce CO₂ whichreduced oil viscosity and this ammonium carbamate when used withsurfactant polymer chemical flood and an enhanced oil recovery of 9.7%was recorded [Shiau, B. J. B., Hsu, T.-P., Roberts, B. L., & Harwell, J.H. 2010. Improved Chemical Flood Efficiency by In Situ CO₂ Generation.Paper SPE 129893-MS presented at the SPE Improved Oil RecoverySymposium, 24-28 April, Tulsa, Okla., USA—incorporated herein byreference in its entirety].

Xiaofei et al. generate in situ CO₂ using active acid, polymer, andsurfactant to solve the problem of reservoir heterogeneity and highwater to oil viscosity ratio which results in monolayer andmonodirectional flow. CO₂ flooding reduced injection pressure andeffectively plugged channeling between injection-production wells. Theirsystem increased the swept volume, increased oil, and decreased watercut [Xiaofei J., Kuiqian M., Yingxian L., Bin L., Jing, Z., and YanlaiL. 2013. Enhanced Heavy Oil Recovery by In-Situ Carbon DioxideGeneration and Application in China Offshore Oilfield. Paper SPE 165215presented at The SPE Enhanced Oil Recovery Conference held in KualaLumpur, Malaysia, 2-4 Jul. 2013—incorporated herein by reference in itsentirety].

U.S. Pat. No. 8,616,294 describes an apparatus for generating carbondioxide gas in situ at an oil site for use in enhanced oil recovery(EOR). The apparatus includes a steam generator adapted to boil andsuperheat water to generate a source of superheated steam, as well as asource of essentially pure oxygen. The apparatus also includes a steamreformer adapted to allow the reaction of a carbonaceous material withthe superheated steam and the pure oxygen, in an absence of air, togenerate a driver gas comprising primarily carbon dioxide gas andhydrogen gas.

US 2014/0231080A1 describes use of acid and sodium bicarbonate togenerate CO₂ in the wellbore itself but not the reservoir. The CO₂ thatis generated in situ will increase the pressure inside the wellbore tolift the oil from the bore.

In view of the above, there remains an apparent need for in situ CO₂generation systems and methods that can reach deep into oil reservoirand sustain the actual reservoir conditions. The present disclosure aimsto provide a method having a different approach towards in situgeneration of CO₂ in carbonate and sandstone reservoirs.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present disclosure pertains to aprocess for recovering hydrocarbon from a carbonate reservoir having aplurality of carbonate geologic formations. The process comprisesinitially flooding the carbonate reservoir with a first portion of afluid to displace and recover a first portion of the hydrocarbon;injecting a dispersion into the carbonate reservoir, the dispersioncomprising a second portion of the fluid and a plurality of citricacid-filled microcapsules; incubating the dispersion in the carbonatereservoir; and

finally flooding the carbonate reservoir with a third portion of thefluid to displace and recover a second portion of the hydrocarbon.During the incubating, the dispersion infiltrates the plurality ofcarbonate geologic formations. The plurality of citric acid-filledmicrocapsules undergoes degradation and releases the citric acid. Thereleased citric acid reacts with the carbonate geologic formations togenerate carbon dioxide at the formations. The generated carbon dioxidecontacts the second portion of the hydrocarbon, swells and reducesviscosity of the second portion of the hydrocarbon.

In some embodiments, the plurality of citric acid-filled microcapsulesundergoes thermal physical degradation, thermal chemical degradation, orboth.

In some embodiments, the initial flooding displaces and recovers up to60% of original oil in place in the carbonate reservoir. The finalflooding displaces and recovers a further 10-35% of original oil inplace in the carbonate reservoir.

In certain embodiments, the citric acid-filled microcapsules are roundand have a diameter range of 0.1-50 μm.

In one or more embodiments, the citric acid filled-microcapsulescomprise an outer polymeric shell and an inner solid core materialencapsulated by the outer polymeric shell. The inner solid core materialcomprises citric acid solid. In one embodiment, the outer polymericshell has the following characteristics: water insoluble; non-porous; nogreater than 2 μm in thickness; free of silicon, phosphorus, fluorine,bromine, chlorine and sulfur; and free of a chemical crosslinker.

In at least one embodiment, the inner solid core material comprises1-20% of the citric acid solid per total weight of the citricacid-filled microcapsule.

In some embodiments, the inner solid core material further comprises1-20% of one or more chelating agents per total weight of the citricacid-filled microcapsule. The one or more chelating agents are selectedfrom ethylenediaminetetracetic acid (EDTA),(hydroxyethyl)ethylenediaminetriacetic acid (HEDTA), glutamicacid-N,N-diacetic acid (GLDA), methylglycine-N,N-diacetic acid (MGDA),nitrilotriacetic acid (NTA), hydroxyethyliminodiacetic acid (HEIDA),iminodisuccinic acid (IDS), polyaspartic acid andethylenediamine-N,N-disuccinic acid (EDDS).

In certain embodiments, the dispersion has a density of 10⁴-10¹² citricacid filled-microcapsules per liter of the second portion of the fluid.

According to a second aspect, the present disclosure relates to aprocess for recovering hydrocarbon from a sandstone reservoir having aplurality of sandstone geologic formations where the injectedmicrocapsules are filled with citric acid and calcium carbonate. Upondegradation of the microcapsules, the released citric acid and calciumcarbonate react with each other to generate carbon dioxide at thesandstone geologic formations. The inner solid core material of themicrocapsule comprises citric acid solid and calcium carbonate solid.

In certain embodiments, the inner solid core material comprises 1-20% ofthe citric acid solid per total weight of the microcapsule and 3-60% ofthe calcium carbonate solid per total weight of the microcapsule.

According to a third aspect, the present disclosure provides amicrocapsule comprising an outer polymeric shell and an inner solid corematerial encapsulated by the outer polymeric shell. The outer polymericshell has the following properties: water insoluble; non-porous; nogreater than 2 μm in thickness; free of silicon, phosphorus, fluorine,bromine, chlorine and sulfur; and free of a chemical crosslinker. Theinner solid core material comprises 1-20% of citric acid solid per totalweight of the microcapsule and optionally 3-60% of calcium carbonatesolid per total weight of the microcapsule.

In certain embodiments, the inner solid core material further comprises1-20% of one or more the aforementioned chelating agents per totalweight of the microcapsule.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a curve showing oil recovery trend when 10 wt. % encapsulatedcitric acid is used on a 6-in Indiana limestone core.

FIG. 2 is a micrograph showing encapsulated citric acid.

FIG. 3 shows and compares oil recovery efficiencies of two processes: aprocess where only sea water is injected and a process where 10 wt. %citric acid microcapsules are injected.

FIG. 4 shows simulated oil recovery factor SCTR versus time in years oftwo oil recovery processes from a waterdrive carbonate reservoir.

FIG. 5 is a bar graph showing interfacial tension reduction of residualoil by encapsulated and non-encapsulated spent citric acid.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views.

The present disclosure will be better understood with reference to thefollowing definitions:

For purposes of the present disclosure, the terms “reservoir”, “oilreservoir” and “petroleum reservoir” refer to a component of a petroleumsystem (i.e. hydrocarbon or petroleum-generating and storing geologicsystem) that is composed of a subsurface body of rock formations havingsufficient porosity and permeability to store and transmit fluids.Sedimentary rocks are the most common reservoir rocks because they havemore porosity than most igneous and metamorphic rocks and form undertemperature conditions at which hydrocarbons can be preserved. Dependingon the type of sedimentary rock, reservoirs can be classified ascarbonate reservoirs having predominantly limestones and sandstonereservoirs having primarily siliclastic rocks and clay. In general,carbonate reservoirs tend to have lower primary permeability andsalinity compared to sandstone reservoirs.

For purposes of the present disclosure, the terms “bore” and “wellbore”refer to a drilled hole or borehole, including the open hole or uncasedportion of the well. In some embodiments, a borehole refers to theinside diameter of the wellbore wall, the rock face that bounds thedrilled hole.

For purposes of the present disclosure, the term “near wellbore” refersto an area or a region that is no more than 10 feet away from thewellbore, preferably 1-8 feet away, more preferably 2-6 feet away.

For purposes of the present disclosure, the term “permeability” refersto the ability, or measurement of a reservoir rock ability, to transmitfluids and is typically measured in darcies (d) or millidarcies (md).Formations that transmit fluids readily, such as sandstones, aredescribed as permeable and tend to have many large, well-connectedpores. Impermeable formations tend to be finer grained or of a mixedgrain size, with smaller, fewer, or less interconnected pores. As usedherein, a “low-permeability reservoir” refers to an oil reservoir havinga range of permeability that is no higher than 10 md, preferably 0.05-10md, more preferably 0.1-7.5 md, even more preferably 0.5-5 md, mostpreferably 1-5 md. Accordingly, as used herein a “high-permeabilityreservoir” refers to an oil reservoir having a range of permeabilitythat is higher than 10 md.

For purposes of the present disclosure, the term “porosity” refers tothe percentage or ratio of void space to the pore volume (PV) of a rock,or that total volume within the rock that can contain or hold fluids,which is typically no more than 20-25% for both sandstone and carbonatereservoirs. “Total porosity” is the total void space in the rock whetheror not it contributes to fluid flow. Thus, effective porosity istypically less than total porosity.

For purposes of the present disclosure, the term “pore volume” refers tothe total volume in a reservoir that can be occupied by fluids. Thisterm is also used as a measurement unit referring to amount of fluidsuch as chemical fluid or water that is injected into a reservoir duringsecondary and tertiary recoveries.

For purposes of the present disclosure, the term “water injection” or“waterflooding” refers to a method of oil recovery in which water isinjected into a petroleum reservoir to sweep and displace mobile oil.The water injected increases pressure within the reservoir, replenishingthe natural reservoir pressure that has been previously depleted inprimary recovery, and physically sweeps the displaced mobile oil to theadjacent production wells. Generally, the water or fluid used in awaterflooding process is taken from nearby water sources, and is usuallynatural sea water, fresh water, produced water (byproduct of the oilindustry), aquifer water, river water, artificial saline water or brine.The water injected during waterflooding has a wide salinity range of500-200,000 ppm, preferably 1,000-100,000 ppm, more preferably2,500-75,000 ppm. Minerals contained in the saline solution include butare not limited to chloride, sodium, sulfate, magnesium, calcium,potassium, bicarbonate, carbonate, bromide, boron, strontium andfluoride. In the present disclosure, the water or fluid injected furtheracts as a medium for carrying the citric acid microcapsules toinfiltrate reservoir rock formations and as an aqueous solvent fordissolving citric acid released from degraded capsules.

For purposes of the present disclosure, the term “thermal physicaldegradation” refers to a process whereby the action of heat or elevatedtemperature on a polymer that causes a change or loss of physical,mechanical and/or electrical properties.

For purposes of the present disclosure, the term “thermal decomposition”or “thermal chemical degradation” refers to a process of chemicalspecies change in a polymer that is caused by heat, which involvesscission of one or more covalent bonds such as but not limited to acarbon-carbon bond (single, double or triple), a carbon-hydrogen bond, acarbon-oxygen bond (single or double), a carbon-nitrogen bond, an esterbond and an amide bond.

For purposes of the present disclosure, the term “glass transitiontemperature” (T_(g)) refers to a temperature range, at which amorphousmaterials such as polymers transition between a hard, brittle, glassystate to a molten, rubbery state. T_(g) is usually expressed as asingular value when the transition between the glassy state and therubbery state reaches 100%. When the polymer is cooled below the T_(g),it becomes hard and rigid. Above the T_(g), the mobility of the polymerchains increase significantly and the polymer assumes a more pliablenature. A glass transition temperature is often measured as the energyreleased on heating by the differential scanning calorimetry (DSC)technique.

The present disclosure provides a process for recovering hydrocarbonfrom a petroleum reservoir. The hydrocarbon contains primarily crude oilbut natural gas is also often present. While this process can be appliedto both sandstone and carbonate reservoirs, the advantages of theprocess can be best manifested when used on carbonate reservoirs withlow permeability. This process is designed to supplement primary(natural) and secondary recoveries, and includes an initial flooding ofa carbonate reservoir with 1-3 pore volumes (PVs) of water which may benatural sea water, fresh water, produced water, aquifer water, riverwater, artificial saline water or brine (waterflooding), followed by atleast a single injection of the same or different water that containsand carries a plurality of citric acid-filled microcapsules. The amountof the citric acid microcapsule dispersion injected is 1-5 pore volumes,preferably 2-5 pore volumes, more preferably 2-4 pore volumes. After anincubation period, the carbonate reservoir is flushed again with up to10 pore volumes of the water without the microcapsules. The initialwaterflooding recovers up to 60% of a reservoir's original oil in place,preferably 5-60%, more preferably 10-50%, even more preferably 20-40%.The final waterflooding that ensues flushing of the reservoir withcitric acid microcapsules and the incubation recovers a further 10-35%of the reservoir's original oil in place. Hence, the oil recoveryprocess provided by the present disclosure recovers up to 90% of theoriginal oil in place of a reservoir, preferably 15-90%, more preferably45-90%, more preferably 55-90%, even more preferably 60-85%, mostpreferably 65-80%. In this process, contrary to known acid treatmentmethods of the wellbore region, the citric acid-filled microcapsules areinjected deep into the oil reservoir.

Injection flow rates of the initial waterflooding, citric acidmicrocapsule dispersion and final waterflooding can be kept constant orvaried, but are no higher than 250 L/s or 0.25 m³/s, preferably 5-200L/s, more preferably 10-150 L/s, even more preferably 50-100 L/s.

Injection pressures of the initial waterflooding, citric acidmicrocapsule dispersion and final waterflooding can also be keptconstant or varied, but are no higher than 10,000 psi, preferably1,000-10,000 psi, more preferably 2,500-8,500 psi, even more preferably5,000-7,500 psi.

The citric acid microcapsule dispersion has a capsule density of10⁴-10¹²/L, preferably 10⁵-10¹⁰/L, more preferably 10⁶-10⁸/L. Factorsdetermining the capsule density include capsule size and amount of insitu generated CO₂ required to effectively reduce oil viscosity and movethe oil.

Advantageously, directional drilling is practiced in well-drillingoperations preceding the oil recovery the production well. Inparticular, the reservoir rock formations are drilled horizontally tocreate at least one horizontal or lateral passage across the reservoir.The horizontal orientation of the reservoir well is configured tomaximize contact between the citric acid microcapsules and theformations. The oil well system at an oil field typically includes atleast one injection/injector well through which fluids and citric acidmicrocapsule dispersion are delivered to the reservoir and at least oneproduction well through which displaced oil is brought to the earthsurface. Certain portions or the entireties of these wells may also benon-vertical, i.e. slant or horizontal.

Once placed deep inside the oil reservoir, the citric acid microcapsulesare left to incubate in the natural reservoir temperature which willtrigger breakdown of the capsules to release the citric acid. Theinjected water protects the microcapsules against mechanical forces andshear stress so that they do not prematurely break down before they areplaced deep inside the reservoir. Additionally, the injected water actsas a medium for transporting the microcapsules to infiltrate thereservoir rock formations and seep through the formation pores. Thereleased citric acid will form a citric acid solution with the injectedwater. The formed citric acid solution contacts and reacts with thecalcium carbonate that is found in abundance in carbonate reservoirrocks to generate CO₂, in situ, at the reservoir:C₃H₄(OH)(COOH)₃(aq)+3CaCO₃(s)→Ca₃(C₆H₅O₇)₂(aq)+3CO₂(g)+3H₂O  (1)In other words, citric acid acts as a CO₂ precursor. The CO₂ generatedhas limited mobility as it is generated and is therefore contained bythe rock formations. The CO₂ generated encounters the residual oiltrapped within the rock formations of the reservoir, causes the oil toswell, reduce in viscosity and move towards to a nearby production well.

Reservoir temperature is primarily governed by the reservoir's proximityto the earth's mantle, and by the relative heat exchange capacities andthermal conductivities of the formations forming the lithostaticsequence that includes the reservoir. The reservoir temperaturetypically ranges from 50-120° C., with an average temperature of 75-85°C. The citric acid microcapsules are incubated for at least 12 h,preferably 12-36 h, more preferably 18-30 h, even more preferably 20-28h, most preferably 22-26 h.

In FIG. 1, a curve representing oil recovery trend from a corefloodingsimulation is shown. The Indiana limestone core used in the simulationhas a core length of 6 in, an initial porosity of 0.15 or 15%, and aninitial permeability of 10.8 md. The fluid injection rate is keptconstant at 0.25 cc/min. The core is injected with 2 pore volumes of seawater, which results in ˜60% of the original oil in place beingrecovered. Then, the core is injected with 2 pore volumes of sea waterwith dispersed citric microcapsules and the core is left to soak in thedispersion for 24 h at 100° C. After that, the core is flushed with 3pore volumes of sea water where a further ˜25% of the core's originaloil in place is extracted.

An image of citric acid-filled microcapsules according to oneembodiment, as acquired by scanning electron microscopy (SEM) ispresented in FIG. 2. A citric acid-filled microcapsule is composed of anouter polymeric shell whose non-porous wall defines the shape and sizeof the microcapsule, and an inner, encapsulated solid core materialcomprising citric acid. The encapsulated citric acid is in anon-reactive solid form, such as powder or pellet, which, once releasedfrom a thermally degraded microcapsule shell, dissolves readily in thewater injected during the waterflooding step to form a reactive citricacid solution. The non-reactive citric acid solid can be anhydrous orhydrated (i.e. monohydrate). The amount of citric acid solid containedin a microcapsule is 1-20% by weight per total weight of a filledmicrocapsule, preferably 1-15%, more preferably 2-10%. In certainembodiments, the microcapsule further contains one or more chelatingagents, also in solid form, at a concentration of 1-20% by weight pertotal weight of a microcapsule, preferably 1-15%, more preferably 2-10%.The one or more chelating agents are selected fromethylenediaminetetracetic acid (EDTA),(hydroxyethyl)ethylenediaminetriacetic acid (HEDTA), glutamicacid-N,N-diacetic acid (GLDA), methylglycine-N,N-diacetic acid (MGDA),nitrilotriacetic acid (NTA), hydroxyethyliminodiacetic acid (HEIDA),iminodisuccinic acid (IDS), polyaspartic acid andethylenediamine-N,N-disuccinic acid (EDDS). In alternative embodiments,the encapsulated citric acid may be mixed or substituted with one ormore of other weak organic acids that can react with calcium carbonateto generate CO₂, which include but are not limited to formic acid,acetic acid, propionic acid, butyric acid, valeric acid, caproic acid,oxalic acid, lactic acid, malic acid, benzoic acid and carbonic acid.

The polymeric shell has a wall thickness that is preferably no greaterthan 2 μm or 2000 nm, preferably 25-1000 nm, more preferably 50-750 nm,even more preferably 100-500 nm, so that the wall can rupture easily torelease the citric acid content when subjected to the reservoir hightemperatures. In one embodiment, the microcapsule is of a generallyspherical or round shape having a diameter range of 0.1-50 μm,preferably 0.2-20 μm, 0.5-20 μm, 1.0-20 μm, 5-20 μm, 10-20 μm, 15-20 μm,more preferably 0.1-10 μm, 0.1-5.0 μm, 0.2-5.0 μm, even more preferably0.1-2.0 μm, 0.2-2.0 μm, 0.5-2.0 μm, 1.0-2.0 μm, most preferably 0.1-1.0μm, 0.2-1.0 μm, 0.1-0.5 μm, 0.2-0.5 μm. In another embodiment, themicrocapsule is oval or rod-shaped where the large diameter (for theoval) or length (for the rod) ranges from 0.5-75 μm, preferably 1.0-60μm, 5.0-50 μm, more preferably 7.5-30 μm, 10-30 μm, 10-25 μm, 15-25 μmwhile the small diameter or width is ranged from 0.05-20 μm, preferably0.1-10 μm, more preferably 0.1-5.0 μm, 0.2-5.0 μm, 0.5-5.0 μm, 0.1-2.5μm, 0.2-2.5 μm, 0.5-2.5 μm. Generally, if a targeted reservoir has lowpermeability, the oil recovery process in accordance with the presentdisclosure would require the citric microcapsules to have a smallersize. The microcapsule can also assume other common shapes such asoblong, bullet, tubular and even less common geometrical shapes likesquare, rectangular, triangular and cylindrical.

To avoid adding undesirable impurities to the extracted hydrocarbon, thepolymeric microcapsule shell is advantageously free of elements such asbut not limited to silicon, phosphorus, fluorine, bromine, chlorine andsulfur. Acceptable elements of the polymer include carbon, hydrogen,oxygen and nitrogen.

To accelerate the degradation rate of a capsule, the polymeric shell isadvantageously free of chemical crosslinking agents. As used herein,“chemical crosslinkers” or “chemical crosslinking agents” are moleculesthat form thermally stable covalent bonds or chemical crosslinks withtwo or more polymer chains in a polymer, to link one polymer chain toanother, in order to promote mechanical strength and thermal stabilityof the polymer. Examples of crosslinkers commonly used in manufacturingof polymers include 4-vinylbenzocyclobutene, trimethylolpropaneethoxylate, triethylene glycol dimethacrylate, poly(ethylene glycol)diacrylate, poly(ethylene glycol) bisazide, 1,4-phenylenediacryloylchloride, pentaerythritol propoxylate, pentaerythritol ethoxylate,1,1-maleimidoundecanoic acid, hexa(ethylene glycol) dithiol, glycerolethoxylate, p-divinylbenzene, divinylbenzene, 1,4-bis(4-vinylphenoxy)butane and bis(2-methacryloyl)oxyethyl disulfide. Molecules that formheat-reversible physical crosslinks such as hydrogen bonds betweenpolymer chains may be acceptable to incorporate into the polymericmicrocapsule shell, but are not required.

The polymeric microcapsule shell is hydrophobic and water-insoluble, orhas limited water solubility at temperatures that are lower thanreservoir temperature.

The citric acid filled-microcapsules of the present disclosure can beprepared by any known fabrication technique suitable formicroencapsulation of solids, and is not so limited. These manufacturingtechniques include but are not limited to pan coating, air-suspensioncoating, fluid-bed coating, spray-drying and congealing, emulsionpolymerization, in situ polymerization, matrix polymerization,layer-by-layer assembly, coacervation and phase separation, solventevaporation, interfacial polycondensation, interfacial crosslinking,centrifugal extrusion, and vibrational nozzle [Ghosh, S. K. (Editor),Functional Coatings: By Polymer Microencapsulation. Wiley, July 2006.ISBN: 978-3-527-31296-2; Jyothi, N. V. N, Prasanna, P. M., Sakarkar, S.N., Prabha, K. S., Ramaiah, P. S., Srawan, G. Y. MicroencapsulationTechniques, Factors Influencing Encapsulation Efficiency. Journal ofMicroencapsulation, 2010, 27(3):187-197—each incorporated herein byreference in its entirety].

The following paragraphs further describe different strategies adoptedin the present disclosure to ensure that the capsule-enclosed citricacid is selectively released inside the reservoir and not prematurely inthe injection well. Importantly, the microcapsule shell is able towithstand the geothermal gradient and shear stress across the injectionwell, as well as the injection pressure.

In one or more embodiments, in addition to the aforementioned polymerproperties and characteristics, the polymeric microcapsule shell is ahomo- or copolymer whose glass transition temperature falls within ±10°C. of the average temperature of a targeted oil reservoir temperaturerange of 50-120° C., preferably ±8° C., more preferably ±5° C. Forexample, if a targeted reservoir has an average temperature of 80° C.,the polymer constituting the microcapsule shell would have a glasstransition temperature of 70-90° C., so that, when subjected to thereservoir temperature, the polymeric shell transitions from an orderedcrystalline or semi-crystalline structure to an amorphous state. Whilein an amorphous state, where polymer chains are randomly oriented, theshell at least partially loses its mechanical strength, collapses andreleases the encapsulated citric acid. Referring to the present example,homopolymers having a glass transition temperature of 70-90° C. andsatisfying the other criteria of being free of sulfur, silicon,phosphorus, halogens and chemical crosslinkers include but are notlimited to poly(tert-butyl acrylate), poly(tert-butyl vinyl ether),poly(cyclohexyl methacrylate), poly(cyclohexyl vinyl ether),poly(4-ethoxystyrene), poly(ethylene terephthalate),poly(2-hydroxypropyl methacrylate), polyindene, poly(isopropylmethacrylate), poly(phenylene vinylene), poly(phenyl vinyl ketone),poly(vinyl pivalate) and poly(vinyl cyclohexanoate). The specific glasstransition temperature values of these homopolymers are given in Table1.

TABLE 1 Glass transition temperatures (T_(g)) of homopolymers.Homopolymer T_(g) (° C.) Poly(tert-butyl acrylate) 43-107Poly(tert-butyl vinyl ether) 88 Poly(cyclohexyl methacrylate) 92Poly(cyclohexyl vinyl ether) 81 Poly(4-ethoxystyrene) 86 Poly(ethyleneterephthalate) 72 Poly(2-hydroxypropyl methacrylate) 76 Polyindene 85Poly(isopropyl methacrylate) 81 Poly(phenylene vinylene) 80 Poly(phenylvinyl ketone) 74 Poly(vinyl pivalate) 86 Poly(vinyl cyclohexanoate) 76

In some embodiments, further to being water insoluble, as well assulfur, silicon, phosphorus, halogen and crosslinker-free, the polymericmicrocapsule shell is constructed of a homo- or copolymer where therepeating units or monomers are connected by non-carbon-carbon covalentbonds that tend to have a lower thermostability, including but are notlimited to ester linkages, amide linkages and glycosidic linkages. Sucha microcapsule shell is susceptible to thermal decomposition orpyrolysis when exposed to the elevated temperatures of a reservoir, asthe aforementioned covalent linkages are cleaved to release theencapsulated citric acid. Examples of homopolymers or copolymers havingester/amide/glycoside linkages include but are not limited topolyglycolide or polyglycolic acid, polylactic acid, polycaprolactone,polyhydroxyalkanoates (e.g. polyhydroxybutyrate), polyethylene adipate,polybutylene succinate, poly(3-hydroxybutyrate-co-3-hydroxyvalerate),polyethylene terephthalate, polybutylene terephthalate, polytrimethyleneterephthalate, polyethylene naphthalate (polyesters), cellulose,cellulose nitrate, cellulose acetate, cellulose diacetate, cellulosetriacetate, polycarbonates and polyamides. In Table 2, chemicalstructures of the repeating units of a variety of exemplary polyesters,cellulose-based polymers, polycarbonates and polyamides are provided.

TABLE 2 Polymers having ester, glycosidic or amide linkages. Linkagebetween repeating Polymer class Examples Repeating unit units PolyesterPolyglycolide or polyglycolic acid

Ester Polylactic acid

Polycaprolactone

Polyhydroxy- alkanoates, e.g. polyhydroxy- butyrate

Polyethylene adipate

Polybutylene succinate

Poly(3- hydroxybutyrate- co-3- hydroxyvalerate)

Polyethylene terephthalate

Polybutylene terephthalate

Polytrimethylene terephthalate

Polyethylene naphthalate

Polysaccharide or cellulose- based Cellulose, Ethyl cellulose, Celluloseacetate, Cellulose diacetate, Cellulose triacetate, Cellulose nitrate

Glycosidic Polycarbonate Bisphenol A, Bisphenol A terephthalate,Bisphenol A carbonate, Bisphenol F, Bisphenol F carbonate, Bisphenol Z,Bisphenol Z carbonate

Ester Polyamide Polyamide 6, Polyamide 6, 6

Amide

In view of the above, it is apparent that heat, specifically an oilreservoir heat, is the main environmental factor inducing release ofcitric acid from the microcapsules, where the polymeric shell undergoesthermal physical degradation, thermal decomposition, or both. The citricacid microcapsules can also be additionally triggered by othermechanisms such as pressure, shear stress, pH and thermal hydrolysis.For example, while oil reservoirs usually have a pH value of 7 or anear-neutral pH, some reservoirs can be acidic or basic. Microcapsuleshaving pH-responsive polymeric shells can be placed in a reservoirhaving the same acidity or alkalinity where the shells collapse andrelease the enclosed citric acid. pH-responsive polymers are those thatcontain an acidic group or a basic in their repeating units, such aspolylactic acid and polyamides.

The present disclosure relates to a process for recovering oil from asandstone reservoir. When targeting a sandstone reservoir, the reservoiris initially flooded with a fluid (i.e. natural sea water, fresh water,produced water, aquifer water, river water, artificial saline water orbrine), then the same or a different fluid containing and carrying aplurality of microcapsules filled with a mixture of citric acid andcalcium carbonate in solid form, i.e. 1-20% of citric acid per totalweight of the microcapsule, preferably 1-15%, more preferably 2-10% and3-60% of calcium carbonate per total weight of the microcapsule,preferably 5-50%, more preferably 10-45%. The sandstone reservoir isthen left to soak in the microcapsule dispersion for a predeterminedperiod to allow rupture of the polymeric microcapsule shells and releaseof the encapsulated citric acid and calcium carbonate. The citric acidand the calcium carbonate react with each other to generate CO₂ in situat the reservoir. After that, the sandstone reservoir is flushed againwith the fluid without any microcapsule. Parameters such as amount offluid/dispersion injected, incubation period, injection flow rate andcapsule density are as described for the oil recovery process from asandstone reservoir.

The present disclosure is further illustrated by the following examplesshowing and comparing recovery efficiencies of different oil recoveryprocesses and interfacial tension values of oil when encapsulated citricacid or non-encapsulated citric acid is used in oil recovery. Theseexamples are not intended to limit the scope of the invention includingthe appended claims.

Example 1

Referring to FIG. 3, where oil recovery efficiencies of two processesare shown and compared. In the first process, 6 pore volumes of seawater are injected into a carbonate reservoir to displace oil. In thesecond process, 6 pore volumes of sea water carrying 10 wt. % citricacid microcapsules are injected into the same carbonate reservoir. Thefirst process utilizing only sea water recovers ˜42% of the original oilin place while the second process utilizing sea water and 10 wt. citricacid microcapsules recovers ˜50% of the original oil in place.

Example 2

Referring to FIG. 4, where simulated oil recovery factor SCTR versustime in years from an industrial scale waterdrive carbonate reservoir isshown. Waterdrive petroleum reservoirs are characteristically bounded byand in communication with aquifers. The solid line represents use of twohorizontal water injectors and a horizontal production well. The dottedline represents use of one horizontal injector of encapsulated citricacid and a horizontal production well. The data was generated usingCMG's STARS thermal simulator.

As shown in FIG. 4, for the initial five years, the recovery factors aremore or less equal for water and encapsulated citric acid injection, butcontinuous injection of encapsulated citric results in more recovery insubsequent years as other mechanisms besides displacement sweep, i.e.slow release of citric acid and in situ generation of CO₂ in thereservoir causes the oil viscosity reduction and increases its mobility.All the injector wells are drilled in the aquifer and in the oil zone.The citric acid microcapsules will be placed in the reservoir throughhorizontal wells and then allowed to degrade, release citric acid whichreacts with carbonate rocks to generate CO₂.

Example 3

Referring to FIG. 5, where measured interfacial tension (IFT) values ofoil when encapsulated citric acid (2 wt. %, 5 wt. %, 10 wt. %) ornon-encapsulated citric acid is used in oil recovery are presented. Alower IFT value indicates lower oil viscosity. It is apparent from thisfigure that use of encapsulated citric acid results in the oil havinglower viscosity. Higher concentrations of encapsulated citric acid leadto even lower oil viscosity.

Thus, the foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. As will be understood by thoseskilled in the art, the present invention may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the disclosure of the presentinvention is intended to be illustrative, but not limiting of the scopeof the invention, as well as other claims. The disclosure, including anyreadily discernible variants of the teachings herein, defines, in part,the scope of the foregoing claim terminology such that no inventivesubject matter is dedicated to the public.

The invention claimed is:
 1. A process for recovering hydrocarbons froma carbonate reservoir having a plurality of carbonate geologicformations, the process comprising: flooding the carbonate reservoirwith a first fluid thereby displacing and recovering a first portion ofthe hydrocarbons, wherein the carbonate reservoir has a permeability nothigher than 10 md and the first fluid is seawater; then injecting adispersion into the carbonate reservoir, the dispersion consisting ofseawater and a plurality of citric acid-filled capsules having anaverage diameter in a range of from 0.1 to 50 μm, wherein the capsulescomprise an outer polymeric shell consisting of a water insolublepolyglycolide that is free of a chemical crosslinker, silicon,phosphorus, fluorine, bromine, chlorine and sulfur; then incubating thedispersion in the carbonate reservoir for at least 12 h; and thenflooding the carbonate reservoir with a third fluid thereby displacingand recovering a second portion of the hydrocarbons, wherein the thirdfluid is seawater; wherein during the incubating: the dispersioninfiltrates the plurality of carbonate geologic formations; theplurality of citric acid-filled capsules undergoes thermal degradationand releases the citric acid; the released citric acid reacts with theplurality of carbonate geologic formations to generate carbon dioxide atthe plurality of carbonate geologic formations; and the generated carbondioxide contacts the second portion of the hydrocarbons, thereby makingthe second portion of the hydrocarbons swell and reducing a viscosity ofthe second portion of the hydrocarbons.
 2. The process of claim 1,wherein: the flooding with the first fluid displaces and recovers up to60% of original oil in place in the carbonate reservoir; and theflooding with the third fluid displaces and recovers a further from 10%to 35% of original oil in place in the carbonate reservoir.
 3. Theprocess of claim 1, wherein the outer polymeric shell is: non-porous;and no greater than 2 μm in thickness.
 4. The process of claim 1,wherein the capsule comprises an inner solid core material thatcomprises from 1% to 20% of citric acid per total weight of the capsule.5. The process of claim 4, wherein the inner solid core material furthercomprises from 1% to 20% of one or more chelating agents per totalweight of the citric acid-filled capsule, the one or more chelatingagents being selected from the group consisting of(hydroxyethyl)ethylenediaminetriacetic acid (HEDTA), glutamicacid-N,N-diacetic acid (GLDA), methylglycine-N,N-diacetic acid (MGDA),nitrilotriacetic acid (NTA), hydroxyethyliminodiacetic acid (HEIDA),iminodisuccinic acid (IDS), polyaspartic acid andethylenediamine-N,N-disuccinic acid (EDDS).
 6. The process of claim 1,wherein the dispersion has a density of from 10⁴ to 10¹² citric acidfilled capsules per liter of the seawater.
 7. The process of claim 1,wherein the permeability is from 0.05 to 10 md.
 8. The process of claim1, which recovers from 45% to 90% of original oil in place of thecarbonate reservoir.